Stress photo-oxidative aging behaviour of polyamide 6

The long-term stress accelerating aging behaviour of polyamide 6 (PA6) was studied by exposure to UV irradiation. The aging behaviour and mechanism were investigated in terms of creep behaviour, mechanical properties, chemical structure, crystallization and orientation behaviour. It was found that the creep deformation of PA6 under stress/UV irradiation was lower than that of the sample aging only under stress, resulting from crosslinking and low mobility of molecules under UV irradiation. The tensile strength of PA6 under stress and stress/UV irradiation substantially increased at the primary aging stage due to stress-induced molecular orientation. The oxidation of PA6 may also be inhibited by orientation, leading to a relatively low content of carboxylic groups. Under UV irradiation, stress accelerates the degradation of PA6, resulting in strengthening UV absorption due to formation of isolated carbonyl groups. The melt temperature and crystallinity both showed an increase with time, which were much higher for the sample aged under stress/UV irradiation than for that aged only under UV irradiation. Wide-angle X-ray diffraction analysis also showed that the orientation factor and crystallinity of PA6 increased with aging time before 16 days, indicating a clear orientation and crystallization of molecules induced by stress. The UV-induced crosslinking reduced the mobility of PA6 chains, resulting in a lower crystallinity and orientation factor of the sample aged under stress/UV irradiation compared with that under stress aging only. Copyright (C) 2011 Society of Chemical Industry